Check the URL. The story appears in Columbia College's alumni magazine. It's as close to proof as you are going to get without convening a court, swearing in witnesses, and subpoenaing records.
Consider how Columbia College would look if the story were shown to be what Rev. Manning maintains it is.
Ooh, the story came out in a Columbia alumi magazine? Surely they could do better than that story from his old off campus roommate. A story that has nothing to do with Obama being at Columbia.